The present invention relates to the processing of signals in a communication system. More particularly, the present invention relates to adaptively demodulating a baseband signal in a communication system as a function of the frequency band in which the corresponding modulated signal was transmitted.
The demand for wireless communication services has been expanding at an extraordinary rate. To accommodate this demand, the radio spectrum available for wireless communication has increased. For example, wireless communication systems, such as cellular telephone systems, now use a mixture of cellular and PCS frequency bands. Thus, as customers travel across the country and receive service from different cellular providers, the cellular customers now, more than ever, require mobile phones that are capable of dual-band operation to assure seamless phone service.
In addition, various modulation methods and multiple access techniques have been employed to further satisfy the high demand for wireless services. Frequency division multiple access, or FDMA, is, of course, the traditional method. Here, the frequency spectrum is divided into a number of radio channels, wherein each channel corresponds to a different carrier frequency. Time division multiple access, or TDMA, is another method in which each carrier frequency or frequency channel in an FDMA system is further divided into time slots, each of which are assigned to a different user. Therefore, each frequency channel can accommodate multiple users. As will be recognized by one of ordinary skill in the art, TDMA and FDMA are employed in the D-AMPS, PDC, and GSM digital cellular systems. If the radio channel is wide enough, multiple users can use the same channel using spread spectrum techniques and code division multiple access or CDMA. IS-95 and J-STD-008 are examples of CDMA standards.
In a digital communication system, information such as cellular speech, e-mail and internet data is represented by a series of binary information symbols. These binary information symbols are then encoded and modulated so that they can be transferred from a sending unit to a receiving unit over a transmission medium, such as a wire, the air, or magnetic tape. For example, in a digital cellular phone system, the bits representing cellular speech are modulated and the resulting waveform is then transmitted from a base station to a cellular mobile phone unit. In order for the cellular customer to hear the speech, the mobile phone must employ a receiver capable of receiving and demodulating the waveform, and ultimately capable of recovering the sequence of binary information symbols to reproduce the speech.
To accomplish this, a digital receiver comprises, among other things, a radio signal processor. The radio signal processor is tuned to the frequency band and carrier frequency of the transmitted signal. This allows the receiver to pick-up the transmitted signal. The radio signal processor then amplifies, mixes, and filters the transmitted signal down to baseband. While baseband typically refers to zero intermediate frequency, it is used herein to refer to a fixed reference intermediate frequency.
The transmission medium, however, can introduce interference and other phenomena that may distort the transmitted signal. For example, the transmission medium may introduce intersymbol interference, multipath effects or co-channel interference. In addition, a frequency error at the receiver may cause the baseband waveform to rotate, thereby affecting the received signal's in-phase (I) and quadrature (Q) components. These effects are not generally removed from the transmitted signal by the radio signal processor. Therefore, in order to recover the desired binary information symbols from the baseband signal, a receiver typically employs a baseband processor.
In dealing with signal interference and distortion, the baseband processor may, in turn, employ an equalizer. An equalizer is a specialized filter that tracks various channel characteristics, for example, the extent to which the waveform is distorted by the transmission medium. In general, the equalizer tracks channel characteristics by periodically obtaining a channel tap. This is accomplished by generating expected received values (e.g., using known or detected symbols). The expected received values are compared to the actual received values, and the difference between what was actually received and what was expected reflects the channel characteristic being tracked. The equalizer can then use this information to adjust the channel tap estimates to compensate for the interference and/or distortion introduced by the transmission medium.
The adaptation rate or the rate at which the equalizer obtains a channel tap is often referred to as the channel tracking step size. The channel tracking step size depends upon how fast the channel is changing. This, in turn, depends upon, among other things, the corresponding frequency band and/or the carrier frequency of the transmitted signal. For example, the higher the frequency band or carrier frequency, the faster the channel characteristics tend to change. However, the radio signal processor removes the carrier frequency, hence, this information is not available to help the baseband processor decide how to properly set the adaptation rate. Consequently, baseband processors are traditionally designed to set the adaptation rate in accordance with worst possible case conditions (e.g., the highest frequency band and/or the highest carrier frequency).
When the worst case conditions are not in fact present, the traditional method for setting the adaptation rate based upon a worst case scenario results in overuse of baseband signal processing or poor tracking performance. Overuse of baseband processing tends to be very time consuming, it results in less accurate binary information symbol detection, and it can rapidly drain the receiving unit's power source (e.g., a battery).
Other solutions have been proposed. For example, U.S. Pat. No. 5,230,007 (Baum et al.) proposes to adapt the channel tracking step size in response to the performance of the channel tracker itself. Such an approach requires added complexity in estimating performance for different tracking step sizes and determining which tracker is performing best. Because performance estimation is not perfect, the best tracking step size is not always utilized. Alternatively, U.S. Pat. No. 5,268,930 (Sendyk et al.) proposes to estimate the speed of the mobile phone to determine channel tracking step size. Again, there is added complexity, because of the need to estimate the speed of the vehicle in which the phone is being transported. Also, inaccurate vehicle speed estimations will result in performance loss.
Therefore, in a digital communication system that employs a relatively large dynamic frequency range, such as a dual-band cellular telephone system, there is a need to improve the efficiency of the receiver. In particular, there is a need to improve the baseband processing function of the receiver, such that the baseband processor can more efficiently compensate for interference and distortion introduced by the transmission medium, more accurately recover the binary information symbols, and reproduce a high speech quality, without placing an excessive drain on the power source of the unit.